Importance of Maintaining Floodplains in the Tropics
Rhonda BlairNortheastern State University, Broken Arrow, Oklahoma

Introduction:

“You can’t fool Mother Nature!” or so a seventy’s margarine commercial stated. You can try as land developers do by “reclaiming” the land she has set aside to hold flood waters as they gather to make a trek to the sea. They bulldoze acres and acres of river bottom land that has forever been able to support and maintain its diversified plant and animal residents. Floodplains are an essential component of a river system. The multiplicity of its inhabitants and the assets it provides for the ecosystem are irreplaceable. What is a floodplain? A floodplain is the area adjacent to a river or tributary that is generally low and flat geomorphology. It changes at the will of the water current leaving sediment, changing path to form lakes, swamps, oxbows, and islands. During inundation, it floods, holding excess water until the current can move the excess downstream to the ocean. As the flood waters spread out over the floodplain it slows, dropping its sediment and leaving valuable nutrient behind. These nutrients support the diverse habitats that river has created. (Kricher) The tropical floodplain magnifies this system. The tropical floodplains are increasingly at risk of human intervention like dams, land reclamation, and pollution. This paper strives to highlight the importance of tropical floodplains but looking into its components: soil and nutrient dispersion, plant and wildlife habitat, and conservation efforts.

Tropical floodplains worldwide are important but for the purposes of this paper the focus will be on the Amazon Basin. The longest river in the world, the Amazon is supported by over one million acres of floodplain. (McCain, 2001) Being feed by diverse set of tributaries, the Amazon River system combines the whitewater, clearwater, and blackwater feeders to nutrient/mineral rich water that nourishes the Basin’s flora and fauna. Whitewater rivers carry mineral rich sediment with intermediate levels of dissolved organic carbons (DOC) and high levels of ions and fine suspended solids (FSS). Blackwater sources which are tannin rich and very clear have high DOC with low levels of FSS and ions. Clearwater rivers are low in DOC, FSS, and ions. As the current mixes the water from the tributaries the Amazon River becomes nutrient rich. As human population, industry, and agriculture increase along the river basin pollution increase. Even with the increase pollution, the Amazon River system still has habitats that are pristine. (Aufdenkampe)

The Amazon River discharges about 4.5 trillion gallons of water a day and carries 165 of all river water in the world annually. With about 1,100 tributaries and 6.5 million km2 of drainage basin, the Amazon Basin controls 40% of South America’s lands. The rainy season in the Basin never ends. As the northern sections start to dry up the southern sections begin a wet cycle. The Amazon River system can fluctuate between 7 and 13 meters annually leaving the floodplain under 10 meters of water. At this depth, flood waters can spread up to 20 km. (Kricher, 192)

Although floodplains are flat geomorphology they are by no means level. Gullies, trenchs, small lakes, oxbows, sandbars, islands, and swamps create diverse habitats for the tropics miriade of plants and wildlife. As the flood waters wait for the current to draw them back in and move out to sea, sediment and nutrients settle out and plants and phytoplankton utilize the dissolved nutrients such as nitrogen. This serves two purposes. Sediment that is washed into the river system from reclaimed land that no longer has plant roots to hold it in place is removed also to reduce nutrients in the water thus preventing buildups farther down stream.

Man’s persistence in pushing back the Amazon Basin’s forest and swamps in the floodplains for farming and pasture is proving futile. In many cases as the sandy soils hold only the nutrients plants need short term because the topical forest and the flood waters of the Amazon river system are consistently replenishing them. Grass pastures established for cattle grazing and cropland are only productive for a few years before producers have to apply commercial ferlizers to compensate. Sediment is washed into the river system from reclaimed land that no longer has plant roots to hold it in place.

Phytoplanton in the Amazon floodplain is a major food source. The varablility of phytoplankton is related more to the rise and fall of flood waters that to the day length and heat since there is little variation. (de Melo, 1999) Phytoplankton and zooplankton are abundant in the tropical floodplain and helps make the Amazon Basin is home to more than 2,400 species of fish. Many fish species have evolved to spawn during the height of the flood in the nutrient dense water. The whitewaters of the upper Amazon river system are nurtient poor. Spawning in the floodplain provides young fish with an abundanct of phytoplankton and zooplankton, arthopods, seeds, plant matter, and fruits. Many fish have adapted to a diet of fruit or seeds which are made available when flood waters invade the varzea and igapo forest. Fish like the tambaqui, which eat the seeds of the rubber tree (Hevea spruceans) whole and then can eliminate it (disperse) up to 5,495 meters away helping to divesify the tree population. (Nuttle, 2011)

Ecological dynamics in the várzea floodplain.
Ecological dynamics in the várzea floodplain.

Credits: where
(Ecological dynamics in the várzea floodplain., 1997)

Water also aids in dispersing tree and plant seeds. Hydrochory is an important means of plant diversification in the amazon Basin. Seeds are not only dispersed downstream but upstream with the help of tidal floods. Tidal inundations disperse seeds to grounds higher than the average flood and aid in a unique selective process. (Moegenburg, 2001) The tidal inundations are above the average 10 meter flood stage. This “flood pulse” trees can go dormant and drop their leaves. Tree roots and stems become waterlogged and small trees and those that are just emerging may die. Some species survive holding their green leaves as they are submersed under 10 meters of clear water as sunlight penetrates all the way to the forest floor. The flood may last up to 210 days. Many plant species have adapted their reproductive cycle to make use of the seasonal flood. Species fruit at the flood peak and fruits or seeds float or sink until the water level drops and conditions are once again favorable to grow. Morphological adaptions also give plant life and edge. The development of stem nodulation or adventitious roots that allows the plant to fix nitrogen in a waterlogged setting, reducing the number of oxygen-consuming cells in the root cortex, and tangential cell walls of the exodermis giving the roots a hydrophobic barrier are some of the unique adaptions made by Amazonian plants. Some plants use anoxia which reduces the use of energy in a crisis.


Plant succession and propagation strategies in várzea floodplains. In terms of reproductive strategies, from the open waters towards the uppermost parts of the topographic gradient, there is an increase in sexual reproduction (represented by the colour intensity of the arrow) and dependence on dispersion mechanisms associated with long-lasting propagules such as dormant seeds.
Plant succession and propagation strategies in várzea floodplains. In terms of reproductive strategies, from the open waters towards the uppermost parts of the topographic gradient, there is an increase in sexual reproduction (represented by the colour intensity of the arrow) and dependence on dispersion mechanisms associated with long-lasting propagules such as dormant seeds.























The 40,000 plant, 3,000 fish, 450 reptile, 1,500 bird, and 500 mammal species living in the Amazon tropics (not counting insects) accounts for one-tenth of the world’s wildlife species. All depend on the on the cycles of the Amazon rainforest and river basin to survive and no one paper can cover it all. Many are at risk due to loss of habitat. The growing population, deforestation for lumber and clearing land for agriculture, use of fertilizers, pesticides, and herbicides along with increase sediment in the water are all contributing to the problem. In a letter to the Nature: International Weekly Journal of Science, Britaldo Silveira Soare-Filho and colleagues wrote:
“By 2050, current trends in agricultural expansion will eliminate a total of 40% of Amazon forests, including at least two-thirds of the forest cover of six major watersheds and 12 Eco regions, releasing 32 ± 8 Pg of carbon to the atmosphere. One-quarter of the 382 mammalian
species examined will lose more than 40% of the forest within their Amazon ranges.”

The letter goes on to say that although government conserve would help prevent 1/3 of the loss that private conservation efforts where need also. (Silveria Soares-Filho, 2006) Loss of tree cover does not only reduce habitat it allows erosion and the introduction of invasive species to the area. Indigenous people have used slash and burn techniques for centuries to grow crops. The difference between that and today’s extensive agricultural reclamation of the land is that after several years when the soil nutrients played out the native people allowed the farm patch to rejuvenate and return to forest. Modern agriculture continues to farm the depleted land and used synthetic fertilizers which can be volatile or runoff polluting the watershed. The Amazon Basin is as old as Mother Nature herself and set in its ways. The ecological balance of the north and south rainy seasons, the timing of the flowering plants, the mixing of the whitewaters, clearwaters, and blackwater rivers to form the great Amazon, and the pulse of the floodwaters all unite to form the greatest, most diverse regions of the Earth.

No Conclusion:

Additional information

BBC Amazon: The Flooded Forest 01


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